One advantage of the TIE model of cheap, mass-produced craft is not even about its role in combat, but its distribution among garrisons: granularity. The Empire didn't just need to mass its forces for decisive battles. In normal conditions it had to disperse them over its entire territory as a matter of maintaining control, not just to react to any threat but also to dissuade potential threats through fear and prestige. It was, of course, not possible to garrison every single system in the galaxy but a widespread presence was sought. This meant there were be massive garrisons on important systems but also microscopic garrisons in the remotest corners of the galaxy (e.g. Dosuun).
Existing forces had to be split to occupy all of those garrisons. A large number of cheap craft is far easier to split than a small number of expensive ones. If, let's say, an ARC-170 is worth as much as 2 TIE Fighters in combat, then on a fleet massed for battle there's no difference between 1000 ARC-170s and 2000 TIE Fighters. But when it's time to man remote outposts on the Outer Rim, it is convenient to have the TIEs as some outposts will only need a few of them. 2 TIEs can go to separate hangars, a single ARC-170 cannot be split in two.